The drain current of a metal-oxide semiconductor (MOS) transistor will increase in some cases in which a parasitic current path exists between drain and source. This part of the drain current is poorly controlled by the gate contact since the current path is located deeper in the bulk substrate away from the gate. It adds to the subthreshold leakage current, leading to increased power consumption.
The amount of punchthrough current depends mainly on potential distribution under the channel. If the depletion area around the drain well extends too far to the source side, the potential barrier between source and drain is lowered and carriers start to move from source to drain. Therefore, punchthrough highly depends on the applied drain voltage and on the source/drain junction depths.
One way to reduce punchthrough effect is increasing the overall bulk doping level. As a result, source/drain depletion regions may become smaller and may not establish a parasitic current path. However, the higher bulk doping decreases mobility.
Some attempts to prevent punchthrough have made use of spatially restricted dopant implantations. For example, the delta doping of FIG. 1A employs a channel region between the source/drain regions 110 having regions 120 of varying dopant concentrations. In FIG. 1B, halo regions 140 interpose the source/drain regions 110 and the substrate bulk 130. In FIG. 1C, pocket implants 150 are implanted adjacent the lower region of the channel. However, these methods also do not provide satisfactory protection against punchthrough without significantly decreasing mobility.